Now I am assuming you researched the parts :)
Question ... does the factory RSD come with different trim pieces ? I know with the aftermarket , you need to slice the trim piece and slide the shoulder belt out .
This sorta happened to me . Ford showed up at my work and offered a 500 or 750 "rebate" if I test drove a Fusion . I was buying my car like the next day . Whatever "paperwork" didn't clear in 24 hours and I got a check a week later for the rebate .
Could it be ? yes .... Is it ? probably not
Mine was about a 5MPH window from 50-55 .
If yours isn't a window , could be bend wheel , unbalanced wheel , ect...
If it's doing it after 8- only, then it's not the issue discussed in this thread at least.